Sept 28(Reuters) - An experimental Amgen Inc drug that targets a specific genetic mutation shrank tumors in just one of 12 patients with advanced colorectal cancer who were given the highest dose in a small, early-stage trial, the company said on Saturday.

The cancer stopped growing in 10 of the patients taking 960 milligrams of AMG510 daily, while one patient had tumor progression, according to Phase I trial data presented in Barcelona at a meeting of the European Society of Medical Oncology (ESMO).

In addition, one of the 10 patients whose colon cancer had initially stabilized later left the trial after the disease worsened.

Amgen had previously reported a much higher response rate for AMG510 in lung cancer. The trial tested the drug against multiple tumor types.

AMG510 could become the first approved medicine targeting a mutated form of a gene known as KRAS. The mutation, KRASG12C, occurs in around 13% of non-small cell lung cancers, 3% to 5% of colorectal cancers and up to 2% of other solid tumor cancers.

Rivals, such as Mirati Therapeutics Inc, are also developing drugs that target KRAS mutations.

Amgen said it has not decided whether to move AMG510 into a larger Phase II colon cancer study as a monotherapy. The U.S. biotech does plan to begin a colon cancer trial this year testing the pill in combination with drugs known as MEK inhibitors that block a protein associated with tumor growth.

"The fact that we saw one response is encouraging," Greg Friberg, head of oncology development at Amgen, told Reuters, adding that "additional work in understanding mutational drivers is ongoing."

AMG510 is part of a growing trend of precision medicines that target specific gene mutations driving cancer regardless of the organ in which the disease originated.

Earlier this month, the company said AMG510 shrank tumors in about half of advanced non-small cell lung cancer patients in the trial. The U.S. Food and Drug Administration has granted “fast track” status to the drug for lung cancer, which could hasten an eventual approval decision.

Early results appear to indicate that the drug is safe, with six out of 76 patients reporting diarrhea or anemia.